If you remove the insert and use the die as a push through die, it resizes the full length of the case. Because it's a push through, the rim gets worked through as well. So if you are working with straight walled cases that have a rim diameter that is the same or very close to the diameter of the case itself, you will potentially get more uniform brass when run through the Bulge Buster. With the .40 S&W and 10mm there could be a benefit, especially if you are using the cases in multiple barrels/chambers.

Just my interpretation of my limited experience on it. Hope it shines some light.